We currently label the 18th century 'notturnos' as 'nocturnes', but they are not typically the same genre. The notturno is much closer to a serenade (and the terms were often used interchangeably in the 18th century), while the term "nocturne" as first used by John Field and later by Chopin & co is typically a solo piano work. Perhaps they should be separated.

In MGG the two notions are also treated as synonymous, nocturne being the more modern one. Mostly used for piano pieces, but also used in other contexts like Brittens"Nocturne for Tenor and a small orchestra" op.60, 1958. Before Field the name nocturne has been used in a compostion of H. Jadin "3 nocturnes ...p. pioanoforte et flute...(1812).

The problem is the following. If you would install a category "Nocturnes", you will find "Notturnos", so called by the composer, which fit in your category of Nocturnes.And vice versa. I see your point, but a division of all Nocturnes and Notturnos in two disjunct subsets seems not to be practicable.

I'm not really suggesting to make a new category for notturnos, but the 18th century notturno is really signifigantly closer to the "serenade" genre than the 19th century "Nocturne" genre. It's unforunate that the terms are so similar but reflect entirely different genres.